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I came across some strange bolts as I am completing my rebuild of a late '68 L-36. I was pretty sure the motor had never been touched when I bought the car in 1985 and it had 73,000 miles. I rebuilt the motor in 1990 and never changed the exhaust bolts. Now that I more focused on doing it "more correctly," I noticed my exhaust manifold bolts were not all the same- besides being 3/8"-16, I had two types:
Headmark "circle A" was 1&5/16" in length from under the head and had normal threads (closer to 1.290") of which I have 8 of these.
Headmark with 6 radial marks and possibly an "H" or "M" was 1&3/16" in length but had unusual threads that has flat valleys instead of cut grooves of which I have 7. The 16th bolt is the stud used for the power steering bracket.
I read about this type of bolt being called a "LOC" bolt that was more difficult to turn and a chevelle guy said these were used by GM on the outer cylinders #1,2,7,8 in his '70 big block.
Mike, from Paragon, said these were after market style bolts that racers used and did not come stock from GM. Stock bolts were the double circle headstamped 1.25" in length.
Who is right??? Left bolt has 6 radial spikes and possible "M" or "H" while right bolt is a circle with "A." Left bolt is slightly shorter but note the shallow valleys compared to the other bolt. Bolt on the right is the circle "A" bolt. The shallow valleys may be more apparent on the top bolt in this pic.
Your friend who owned the Chevelle is correct. The outer BB exhaust manifold bolts were special bolts that had a slight interference to prevent loosening.
GM used a lot of suppliers for bolts so multiple head marks can be found on most applications. Your circle A bolt was on of them used during this time period on exhaust manifolds.
That's very interesting, considering I cannot find any info on these bolts and none are offered (that I can find) in a set claiming to be "correct!"
I wonder if others have seen these bolts as well and just tossed them because they don't fit nicely by hand...
Thanks, John
Yeah, they are "correct" as long as you ignore the end bolts. I've never seen the end bolts reproduced, so originals are the only way to go if you want that.
Richard Fortier is the man to go to if you need an original bolt. Warning, it won't come cheap.
Thanks Chris-
isn't it interesting that they thought it was important to add to the outer cylinders, yet they did not make a stud with those threads for the power steering bracket! Where is the logic for that?
After a bit of research, these special flat valley "shoulder" bolts are available still. Some are on ebay in original NOS packaging, and another is a well known fastener site "AMKproducts.com and the link is https://www.amkproducts.com/master-k...element=Engine
which shows a picture of the bolts (16) in their master kit for $22 and $10 for shipping. Maybe this info will others... If you look closely, the first groove is cut deeper to get the bolt started.
BB Exhaust manifold experts...
